1. Field of the Invention
This invention relates to the diagnosis and therapy of Epstein-Barr virus associated disease. More specifically, these modalities are founded on the discovery of EBV-specific peptides.
2. Description of Related Art
Epstein-Barr virus (EBV) is a human herpesvirus which is endemic in all human populations. Most people are infected with the virus in early childhood and then carry the virus for life. If the initial infection is delayed until adolescence, infectious mononucleosis (IM) frequently results. The virus is also linked with certain kinds of cancer. In the malarial belt of Africa, EBV is a contributory factor in the development of Burkitt's lymphoma and in South-East Asia, the virus is linked to the high incidence of undifferentiated nasopharyngeal carcinomas.
Acute viral infection leads to the production of specific nuclear antigens (termed EBNA-I and EBNA-II), an “early antigen” (EA) complex, viral capsid antigens (VCA), and other associated molecules. The “early antigen complex” consists of the “early antigen-diffuse” (EA-D) and the “early antigen-restricted” (EA-R) antigens, based on their distribution in immunofluorescence assays in the cytoplasm plus nucleus (i.e. diffuse distribution) or in the cytoplasm only (i.e. restricted) and based on their staining appearance in methanol-fixed cells. These EA antigens, with molecular weight 50-55 Kd, 17 Kd, and 85 Kd, respectively, are synthesized during the “lytic” phase of EBV infection and not in transformed lymphoblastoid cells. Antibodies against the early antigens are present during acute EBV infection and then disappear as the virus enters a phase of latency. The reappearance of anti-EA antibodies signals viral reactivation and provides a clue to the possible role of this virus in diseases such as nasopharyngeal carcinoma and Burkitt's lymphoma.
Indirect evidence has suggested a possible role for EBV reactivation in patients with Sjogren's syndrome, an autoimmune disorder characterized by lymphoid infiltrates of the salivary gland (the normal site for EBV latency). Since antibodies to EA antigens are detected by immunofluorescence assays, such antibodies cannot be detected in patients who possess antinuclear and anticytoplasmic antibodies as part of an autoimmune disease. Therefore it would be desirable to have purified EA molecules to allow measurement of anti-EA antibodies in patients with autoimmune diseases and to more accurately quantitate anti-EA antibodies in other patients with acute or reactivated EBV.
Recently, the DNA sequence of EBV was determined (Beer, et al., Nature 310:207, 1984) and the EA-D antigen localized in the genome. Using a monoclonal antibody directed against the EA-D protein, sufficient protein was purified to allow partial amino acid sequence determination and thus localization of the coding sequences. Using that information, it was possible to prepare a series of synthetic peptides based on the DNA sequence. The same strategy has proved useful in identifying immunologically important epitopes on the EBNA-I antigen (Rhodes, et al., J. Immunol., 134:211, 1985) and the EBNA-II antigens (Dinner, J. Proc. Natl. Acad. Sci. U.S.A. 81:4652, 1984) of EBV. A synthetic peptide derived from the EA-D molecule which contains an epitope reactive with immune human sera from patients with IM and other disease states has also been described (Fox, et al., J. Clin. Lab. Anal., 1: 140, 1987).
Recent studies have shown that chemically synthesized polypeptides corresponding to short linear segments of a protein's primary amino acid residue sequence can be used to induce antibodies that immunoreact with the native protein (Lerner, et al., Nature, 299:592, 1982; Sutcliffe, et al., Science, 219:260, 1983). In addition, some studies have shown that synthetic polypeptides can immunoreact with antibodies induced by native proteins (Rhodes, et al., J. Immunol., 134:211, 1985). Thus, some synthetic polypeptides can immunologically mimic the immunogenic and antigenic determinants of native proteins.
However, as is well known in the art, the application of synthetic peptide technology still suffers several shortcomings. For instance, the identification of peptides capable of mimicking antigenic determinants on a native protein requires knowing the amino acid residue sequence of the protein. Whereas the amino acid residue sequence can be predicted from the nucleic acid sequence of the gene coding for the protein, such a prediction can only be made if the correct reading frame of the gene is known.
The nucleic acid sequence of the EBV genome is known. However, even if a protein's amino acid residue sequence is known, methods for identifying the loci in the protein that constitute the immunogenic and antigenic determinants are experimental in nature and do not yield predictable results. There are at least two reasons for this. First, without knowing a protein's 3-D structure there is no reliable method for determining which linear segments of the protein are accessible to the host's immune system. Second, whether the 3-D structure is known or not, short linear polypeptides often appear not to have the ability to mimic the required secondary and tertiary conformational structures to constitute appropriate immunogenic and antigenic determinants (Tamer, et al., Nature, 312:127, 1984). However, methods such as Berzofsky's algorithm (AMPHI program, 1987) have been developed which allow the identification of those dominant epitopes of a molecule that preferentially interact with T or B-cells.
Previous studies have examined the cellular immune response to EBV-induced antigens synthesized during the virus replication cycle (Pothen, et al., Int. J. Cancer, 49:656, 1991). The results demonstrated that some of the components of the early antigen (EA) complex were very effective in inducing a strong T-cell proliferative response similar to that previously noted with the major membrane glycoprotein, gp350/250 (Ulaeto, et al., Europ. J. Immunol., 18:1689, 1988). Both CD4+ and CD8+ lymphocyte populations from EBV-infected donors proliferated in the presence of polypeptides purified from the EA complex by immunoaffinity chromatography. The major polypeptide of EA-D and one of the major polypeptides of EA-R were particularly effective in this T-cell recognition assay. The data suggested that these components of the EA complex might function as important target antigens in the immunosurveillance of EBV-infected or immortalized cells. Identification of the dominant T and B-cell epitopes expressed on EA-R complex polypeptides would provide information on the importance of the antibody responses to these components in the diagnosis and management of individuals with EBV-associated lymphoproliferative diseases.
It would be desirable to develop improved methods to assay for the presence of EA-R or EA-D and anti-EA-R or anti-EA-D antibodies in a body sample so as to allow diagnosis of EBV involvement in disease, as well as diagnosis os the stage of a disease such as infectious mononucleosis. The identification of those B and T-cell epitopes on EA-R/EA-D polypeptides would be an important step toward synthesis of the molecules for use for diagnostic and disease management purposes in individuals with EBV associated lymphoproliferative diseases.